Nuclear magnetic resonance (NMR) imaging and spectroscopy have been clearly established as important analytical procedures. Attention is being directed to the possible use of chemical shift resolved (C.S.R.) imaging to provide "maps" which show the spatial distribution of individual chemical species within an object. Such measurements generally require the homogeneity of the magnetic field to exceed 1 part in 10.sup.7 over the region of interest. This stems from the fact that the line widths of the single quantum (SQ) coherence are directly proportional to the overall field homogeneity.
Magnetic field inhomogeneity imposes limitations upon NMR spectroscopy and imaging in two specific areas: high resolution spectroscopy in isotropic liquids, and chemical shift resolved NMR imaging in isotropic liquids. In both cases, magnetic field inhomogeneity may degrade the resolution of spectra to such an extent that no useful information can be obtained from them. In high resolution NMR spectroscopy it is necessary to be able to extract accurately the parameters present within the spectrum such as chemical shifts, coupling constants and peak areas. In chemical shift resolved imaging experiments the requirements are less stringent; and it is only necessary that the resonances of different chemical species be resolved. However, even the less stringent requirements of NMR imaging are often difficult to meet as the sample volumes required are often several orders of magnitude larger than those required in conventional high resolution NMR spectroscopy.
Present NMR methods make use of single quantum, NMR coherences, which have the intrinsic property that their line-widths are directly proportional to the field-inhomogeneity. As a result, they require magnets with homogeneity of 1 part in 10.sup.5 for NMR imaging, 1 part in 10.sup.7 for chemical shift resolutions, and 1 part in 10.sup.8 for coupling constant measurements. All such magnets are expensive. For a field-inhomogeneity of only about 1 part in 10.sup.3, conventional NMR methods would produce only a single broad resonance with absolutely no information content whatsoever.
Single-quantum (SQ) NMR involves processes in which a single quantum transition of a single proton is involved; for example, the proton of CHC1.sub.3 can undergo such a transition. In contrast, zero quantum (ZQ) NMR effectively involves no net change in the spin state of a set of two-protons. Thus CHC1.sub.3 cannot give a ZQ response whereas CHC1=CCHBr can. In effect, each upward transition of one proton, is cancelled by a downward transition for the second.